1. Field of the Invention
This invention relates to a device and a method for accurately locating the origination position of a gamma ray in a diagnostic imaging environment.
2. Description of Related Art
Recent advances in diagnostic imaging, such as magnetic resonance imaging (MRI), computerized tomography (CT), single photon emission computerized tomography (SPECT), and positron emission tomography (PET) have made a significant impact in cardiology, neurology, oncology, and radiology. Although these diagnostic methods employ different techniques and yield different types of anatomic and functional information, this information is often complementary in the diagnostic process.
Generally speaking, PET involves the detection of gamma rays in the form of annihilation photons from short-lived positron emitting radioactive isotopes including, but not limited to .sup.18 F with a half-life of approximately 110 minutes, .sup.11 C with a half-life of approximately 20 minutes, .sup.13 N with a half-life of approximately 10 minutes and .sup.15 O with a half-life of approximately 2 minutes, using the coincidence method. These radiotracers or radiopharmaceuticals typically are synthesized from labeled precursors and are inhaled or injected into the patient.
Typical PET scanners or tomographs consist of banks of large numbers of scintillation detectors surrounding the patient and are coupled to complex computerized data acquisition systems. The images of the temporal and spatial distributions of the inhaled or injected radiopharmaceuticals are reconstructed by using mathematical imagery construction techniques similar to those applied in computerized tomography. PET provides unique functional information on blood flow and metabolism not easily obtainable by other technologies. Because of the short half-lives of the isotopes used, they are typically produced in an on-site cyclotron or other type of particle accelerator.
SPECT, on the other hand, uses longer-lived isotopes including but not limited to .sup.99m Tc with a half life of approximately 6 hours and .sup.201 Tl with a half life of approximately 74 hours. However, the resolution in present SPECT systems is even lower than that presently available in PET systems.
Typical detectors in presently available PET and SPECT systems comprise inorganic scintillators, such as sodium iodine or bismuth germanate scintillators. These scintillators typically offer a resolution of about 1 centimeter (cm) or slightly better. Further, inorganic scintillators can produce pile-up problems, when multiple events occur within a brief period of time within the scintillator due to the slow time resolution of the scintillator. An event comprises the interaction of a particle, including but not limited to a gamma ray, with the scintillator producing photons within the scintillator. A fraction of the photons produced within a scintillator reach a photon detector coupled to the scintillator where they are counted.
Present PET and SPECT systems, due to their limited resolution, are unlikely to show the beginning of blockage in an artery approximately one-half centimeter wide. Similarly, where an isotope injected in a patient attaches to a tumor, presently available systems are typically able to locate the tumor only to an approximate degree of accuracy of about 1 centimeter or slightly better. Particularly where such a tumor is positioned in a sensitive area, such as a brain, a higher spatial resolution system would be desirable to allow more precise location of the tumor. Similarly, increased resolution in PET and SPECT systems could allow for detection of early stages of potential artery blockage along with other benefits generally resulting from improved imaging.